Because of the availability of xerographic and electrostatic office copying machines, the ease with which they can be operated, and the low cost of copying, there has been, in recent years, widespread unauthorized and indiscriminate copying of documents. Unauthorized copying of copyrighted works, such as musical scores and text material, is of particular concern because it violates the legal rights of authors and publishers.
Representative teachings addressing the foregoing problem along with other related prior art are found in U.S. Pat. No. 3,852,088 issued on Dec. 3, 1974, U.S. Pat. No. 3,887,742 which issued on June 3, 1975, U.S. Pat. No. 3,603,681 issued on Sept. 7, 1971, U.S. Pat. No. 3,807,852 issued on Apr. 30, 1974, U.S. Pat. No. 3,713,861 issued Jan. 30, 1973, U.S. Pat. No. 3,671,451 issued on June 20, 1972, U.S. Pat. No. 776,470 issued on Nov. 29, 1904, U.S. Pat. No. 776,515 issued on Dec. 6, 1904, U.S. Pat. No. 17,473 issued on Jan. 2, 1957, U.S. Pat. No. 4,025,673 issued on May 24, 1977, U.S. Pat. No. 3,597,082 issued on Aug. 3, 1971, U.S. Pat. No. 3,977,785 issued on Aug. 31, 1976, U.S. Pat. No. 1,692,405 issued on Nov. 20, 1928, Swiss Pat. No. 109,040, French Pat. No. 809,379, British Pat. No. 402,028, British Pat. No. 411,178, and British Pat. No. 198,364.
Most of the solutions proposed in the foregoing patents contemplate a special form of document which is intended to impair or prevent ligible reproduction. Such solutions, in general, are unsatisfactory mainly because they favor just one type or one small class of copying machines or photoreceptors over others, or are costly, or both. Accordingly, no optimum solution has been found for economically defeating all or most of the different types of popular copying machines presently in commercial use.
For the most part, present day plain paper copying machines (i.e., those utilizing plain copy paper) use one of four different photoconductor or photoreceptor materials with varying mixtures of color sensitizers. Such photoconductor materials are selenium as employed in some Xerox machines, zinc oxide as employed in Royal or Royfax machines, cadmium sulfide as employed in Saxon machines and organic (e.g., polyvinyl carbazole) materials as used in IBM machines. None of the known anti-copying document color systems is capable of adquately preventing the production of intelligible copy of all four classes of photoconductor materials just mentioned.